task combiners: allow custom inputs to the `finalize_funs`

Use-case for arguments being passed to the combiner functions other than the list of files to combine.

The `vizstorm-GIF` use-case is not perfect for task combiners since it needs to combine resulting frames from multiple makefiles instead of combining all the resulting frames from one makefile. However, if we had not added `intro` and `outro` frame makefiles, then I would have been able to use task combiners and my `finalize_funs` would have been `combine_animation_frames`, which is the function that creates the GIF. It is important for this function to be able to accept additional arguments (such as framestep) that can be parameterized using `viz_config`. This is currently not possible with how `finalize_funs` is implemented.